Many diseases in the human body can be detected by analyzing the flow of blood through tissue. For example, tumors are often characterized by tissue having little or no blood flowing into them.
One commonly used technique to analyze the blood flow is ultrasound imaging. While this technique works well for producing images of fast moving blood flow, such as that found in arteries or major veins, it is difficult to produce images of blood moving in capillaries or small vessels because the echo signals produced by the individual blood cells are small in relation to the echoes produced by surrounding tissue and because the blood is moving too slow to accurately image using Doppler techniques.
To enhance the ability of slowly moving or stationary blood to produce strong echo signals, it is often advantageous to introduce a contrast agent into the patient. Typically, contrast agents are liquids containing small gas bubbles that are surrounded by a coating or shell. These bubbles produce stronger echo signals when subjected to ultrasound pulses than the blood cells do. While the use of a contrast agent increases the ability of an ultrasound machine to produce images of blood, it is still difficult to resolve blood in capillaries due to the small amount of contrast agent present at a particular location in the body. In addition, the current techniques for detecting the contrast agent are sensitive to motion artifacts caused by movement of the body or of the ultrasound probe.
Given the above shortcomings in the art, there is a need for a new ultrasound imaging mode that can accurately capture images of slowly moving blood flow or stationary blood.